Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
The oxidative coupling of olefinic compounds and aromatic compounds, to produce olefinically substituted aromatic compounds, is well known, and may be simplistically represented by the following reaction: ##STR1##
A common example of oxidation coupling is the reaction of ethylene and benzene, to produce styrene, as shown below. ##STR2##
The direct oxidative coupling of olefinic compounds and aromatic compounds in the presence of both stoichiometric and catalytic quantities of Group VIII metal salts has been demonstrated in the prior art. See, for example, Shue, 1973, and Intille, 1974. In this context, the Group VIII metals are iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. Heretofore, the most preferred Group VIII metal has been palladium(II), usually provided in the form of a palladium carboxylate, for example, as palladium(II) acetate.
In non-catalytic direct oxidative coupling of olefinic compounds and aromatic compounds, a stoichiometric quantity of the Group VIII metal, as oxidant, is required. During the reaction, the Group VIII metal is reduced and rendered unreactive towards the coupling reaction. In many cases, the reaction can be made catalytic with respect to the Group VIII metal by the addition of a oxidizing agent, which converts the spent (reduced) Group VIII metal back into a reactive (oxidized) form, thereby regenerating the catalyst. Examples of oxidizing agents which have been used in this fashion include iodine, PbO.sub.2, Ag.sub.2 O.sub.2, and Cu(II) salts. If a copper(II) salt is employed, the reaction can further be made catalytic with respect to copper by the introduction of molecular oxygen, which converts spent copper back to copper(II). However, the reaction with molecular oxygen usually requires activation, for example, by a carboxylic acid such as acetic acid, which may be used as the solvent or as a co-solvent.
Despite a possible improvement in reaction rate, the use of a carboxylic acid, such as acetic acid, as a solvent or co-solvent, has several disadvantages. Carboxylic acids are often reactive towards the olefinic compound under the reaction conditions employed, leading to a substantial quantity of undesired by-products. For example, the reaction of benzene and ethylene in the presence of acetic acid typically yields a substantial quantity of vinyl acetate, by the following reaction: ##STR3##
Often twice as much vinyl acetate as styrene is produced in the reaction between benzene and ethylene using a palladium(II) acetate catalyst. This disadvantage has been recognized (see, for example, Shue, 1973) and a solution proposed: use a catalytic amount of palladium carboxylate, a large excess of aromatic compound, in the presence of molecular oxygen at relatively low reaction temperatures (see, for example, col. 7, line 72 through col. 8, line 32 of Shue, 1973).
Applicants have discovered an altogether different solution to the problem. Applicants have discovered that the use of a different catalyst, specifically a rhodium(III) acetylacetonate catalyst in conjunction with a copper(II) redox agent, in a reaction medium which does not comprise a carboxylic acid, results in a comparable catalyst turn over frequency as well as a substantially higher selectivity for the desired product.